The recent discovery of superconductivity in a (La, Ba) cuprate led to worldwide research activity which quickly resulted in the discovery of other metal oxides having relatively high superconductive transition temperature T.sub.c. In particular, it was discovered that YBa.sub.2 Cu.sub.3 O.sub.7 can have T.sub.c of abut 90K. To date, the research efforts have resulted in the identification of several classes of high T.sub.c oxide superconductors.
One class of metal oxide superconductors has nominal composition Ba.sub.2-y (M.sub.1-x M'.sub.x).sub.1+y Cu.sub.3 O.sub.9-.delta., where M and M' are chosen from Y, Eu, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb, Lu, La, Sc, Sr or combinations thereof. Typically, 0.ltoreq.x.ltoreq.1,0.ltoreq.y.ltoreq.1, and 1&lt;.delta.&lt;3. This class will be referred to as the Ba-cuprate system. Many members of the Ba-cuprate system have T.sub.c greater than 77K, the boiling point of liquid nitrogen. Exemplary of the Ba-cuprate system are YBa.sub.2 Cu.sub.3 O.sub.7 (frequently referred to as the "1:2:3" compound), EuBa.sub.2 Cu.sub.3 O.sub.7, and La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.7. (It will be understood that chemical formulae of oxide superconductors and other mixed metal oxides herein are approximate only, and that deviations may occur. For instance, the optimal oxygen content in YBa.sub.2 Cu.sub.3 O.sub.7 frequency is not 7 but about 6.9.)
Other classes of oxide superconductors are the Tl-Ba-Ca-Cu oxides (exemplified by Tl.sub.2 Ba.sub.2 CaCuO.sub.8, see, for instance, Nature Vol. 332, Mar. 31, 1988, page 420), the Bi-Sr-Ca-Cu oxides (exemplified by Bi.sub.2.2 Sr.sub.2 Ca.sub.0.8 Cu.sub.2 O.sub.8+.delta. ; ibid, page 422), and the copper-free bismates (exemplified by Ba.sub..6 K.sub..4 BiO.sub.3 ; see U.S. patent application Ser. No. 187,098, filed Apr. 28, 1988).
A multitude of applications for the novel high T.sub.c oxide superconductors have been proposed, and many of the proposed applications require the formation of bulk bodies comprising superconductive material. For a general overview of some potential applications of superconductors see, for instance. B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDS and MACHINES, Plenum Press 1977; and S. Foner and B. B. Schwartz, editors, Superconductor Materials Science, Metallurgy, Fabrications, and Applications, Plenum Press 1981. Among the applications are power transmission lines, rotating machinery, and superconductive magnets for e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage, as well as junction devices, interconnects, and detectors, It is expected that many of the above and other applications of superconductivity would materially benefit if high T.sub.c superconductive material could be used instead of the previously considered relatively low T.sub.c materials. See also U.S. patent applications Ser. No. 036,160, 126,083, and 022,229, all incorporated herein by reference.
In general, bulk superconducting bodies (including relatively thick films such as are produced by application of a paste to a substrate) are made from a starting material that comprises a powder of the appropriate metal oxide or oxides. Similarly, bulk bodies of non-superconductive metal oxides (e.g., ferrite bodies) can be produced from a starting material that comprises powder of the appropriate oxide or oxides. In all these cases the overall metal composition of the starting material corresponds directly to the desired metal composition of the body to be produced, but the powder can be a mixture of metal oxides. The oxygen content of the powder typically does not correspond to the desired final oxygen content.
Production of high T.sub.c metal oxide powder is frequently accomplished by reaction of the component oxides and/or carbonates, involving typically repeated calcining and comminution steps, which is not only time consuming but may lead to the introduction of contaminants (it is well known that the presence of many common elements leads to poisoning of the superconducting properties such as high T.sub.c). However, other approaches, (including co-precipitation) have also been used to produce the metal oxide powder for bulk superconductors. For instance, J. G. Bednorz and K. A. Muller, Zeitschrift fur Physik B-Condensed Matter, Vol. 64pp. 189-193 (1986) report on page 190 the preparation of multiphase (Ba, La) cuprate by means of co-precipitation from an aqueous solution of the nitrates, using oxalic acid as the precipitant, and heating of the oxalate precipitate. See also A. Manthirama et al, Nature, Vol. 329, Oct. 22, 1987, page 701.
Chung-Tse Chu et al, Journal of the American Ceramic Society, Vol. 70(12), 1987, C-375, report the preparation of high T.sub.c superconducting oxide powder by a citrate process which involves dissolving in water the nitrate slats of the metal constituents of the oxide, adding citric acid to the solution, optionally adding NH.sub.3 OH to bring the pH of the solution to about 6, heating the solution to evaporate the water, and firing the resulting residue to form the desired oxide.
X. Z. Wang et al, Solid State Communication, Vol. 64(6), 1987, page 881 report on the formations of high T.sub.c oxide powder (YBa.sub.2 Cu.sub.3 O.sub.x) by a process that comprises dissolving copper acetate and yttrium nitrate in water, dissolving barium hydroxide in acetic acid, and pouring the resulting solution into an aqueous solution of oxalic acid. The resulting precipitate is then fired to produce the oxide.
Oxalic acid co-precipitation of aqueous metal nitrate solutions is also reported by A. Koyanagi et al, Seissan Kenkyu (Japan), Vol. 39(11), 1987, page 8, M. Hirabayashi et al, Japanese Journal of Applied Physics, Vol. 26(4), 1987, page L454, K. Kaneko et al, ibid, Vol. 26(5), 1987, page L734, D. W. Capone et al, Applied Physics Letters, Vol. 50(9), 1987, page 543, and by T. Kawai et al, Japanese Journal of Applied Physics, Vol. 26(5), 1987, page 736.
R. J. Clark et al, High-Temperature Superconducting Materials, edited by W. E. Hatfield et al, University of North Carolina, pp. 153-158) report, inter alia, treating "1:2:3" solutions of the nitrates with about 75% excess of oxalic acid to precipitate the bulk of the metals, evaporating the suspension to dryness, and heat treating the resulting solid. The procedure is reported to result in an exothermic reaction between nitrate and oxalate. The authors caution that this heat treatment should be done with care using limited quantities of material.
The prior art methods of forming mixed metal oxide power material typically are time consuming and generally not easily adapted to continuous processing and/or scaling up to industrial scale. Furthermore, prior art co-precipitation methods typically depend on control of a variety of parameters such a pH and solubility product.
In view of the potential importance of bulk high T.sub.c metal oxide superconductors and other articles that comprise mixed metal oxide powder, it would be highly desirable to have available a simple, fast, inexpensive, easily controllable, and efficient method for producing mixed metal oxide powder that is suitable for scale-up to industrial quantities. Desirably such a method also would be able to produce essentially single phase material that is essentially free of contaminants and has relatively uniform and relatively small particle size. Furthermore, such a method desirably would have broad applicability and permit the production of a wide range of compositions, including for high T.sub.c superconductive oxides. This application discloses such a method.